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74mm Crank Questions

3346 Views 17 Replies 8 Participants Last post by  Rycooled, 
Rycooled
I have brought a 74 mm crank for my new engine build, im going to be running it with 1641 b & p's, will I need spacers? and if I do, what size do I need?
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Beetlebank
What deck height and compression do you intend to run?
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Rycooled
Beetlebank said:
What deck height and compression do you intend to run?
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I havent a clue at the moment, I assume you'd decide this based on the spec of the engine? This is the first engine i've ever built so im a complete begineer!
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Rycooled
is there somewhere I can find a bit more info on this to have a read up?
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Phil69
Probably best to swap the crank for a 69mm (standard) stroke crank as you will more than likely have to change/adjust valve train geometry due to the slight increase in the overall width of the engine (5mm).
You will need 2.5mm barrel spacers if you decide to go ahead with the build.
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Rycooled
Phil69 said:
Probably best to swap the crank for a 69mm (standard) stroke crank as you will more than likely have to change/adjust valve train geometry due to the slight increase in the overall width of the engine (5mm).
You will need 2.5mm barrel spacers if you decide to go ahead with the build.
Click to expand...
Thankyou for your help! If I was to go ahead, what would I need to do to adjust the valve train geometry? This whole build is a learning process for me, so if it means I get to learn how to do something else I'm more than up for doing it!
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Rycooled
I am assuming that it isn't as simple as just using pushrods that are an extra 2.5 mm longer than stock to make up the difference?
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Phil69
It might be as your only increasing the width of the block by 5mm.
Longer push rods and maybe shims under the rocker bars.
I have limited knowledge of performance engine builds. It might be an Idea to Pm Spanner manager or Beetleragtop these guys along with others have been supplying and racing these beasts for a long time.
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C
Ive built a 1600 with a 76 counterweighted crank so the engine is now a 1745cc stroker, i had to clearance the case aswell, so you may gave to with the 74 crank. Also you will need to get longer pushrods (ask beetleragtop on here for some, v cheap) which you will have to cut to suit your engine, i have a link somewhere about how to do this properly. Its alot more hassle than building a standard engine, but im glad i did it as the engine pulls like a train and is brilliant in my bus.
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Rycooled
Phil69 said:
It might be as your only increasing the width of the block by 5mm.
Longer push rods and maybe shims under the rocker bars.
I have limited knowledge of performance engine builds. It might be an Idea to Pm Spanner manager or Beetleragtop these guys along with others have been supplying and racing these beasts for a long time.
Click to expand...
Awesome, ill give one of them a message and see where that takes me, thanks for your help!
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Rycooled
matthew_g_l said:
Ive built a 1600 with a 76 counterweighted crank so the engine is now a 1745cc stroker, i had to clearance the case aswell, so you may gave to with the 74 crank. Also you will need to get longer pushrods (ask beetleragtop on here for some, v cheap) which you will have to cut to suit your engine, i have a link somewhere about how to do this properly. Its alot more hassle than building a standard engine, but im glad i did it as the engine pulls like a train and is brilliant in my bus.
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I've seen varying opinions on whether ill have to Clearance my case with a 74 mm crank. If you could find that link that'd be awesome!
Cheers man!
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beetleragtop
Rycooled said:
I've seen varying opinions on whether ill have to Clearance my case with a 74 mm crank. If you could find that link that'd be awesome!
Cheers man!
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You may get lucky with the stock rods not needing the case clearing but in my experience you normally need to clear it, however if you get the cb unitech rods it will drop straight in

Longer cut to length pushrods will be needed again unless your lucky

The barrel shims will depend on your desired compression and deck height, with a 5mm bigger stoke to retain stock compression you'd use 2.5mm spacers per side, if you wish to raise your dek and hence compression you would run smaller spacers to set your deck tighter etc

Cb have an engine calculator on their website to work it all out for you

If you get stuck drop me a pm:D

Cheers James
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Forsh
good read...


http://www.amazon.co.uk/Aircooled-Engine-Interchange-Motorbooks-Workshop/dp/0760303142

:)
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Mr.Whippy
Also a good read - http://johnmaherracing.com/jmr-performance-guide-cd/

Dave.
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Rycooled
beetleragtop said:
You may get lucky with the stock rods not needing the case clearing but in my experience you normally need to clear it, however if you get the cb unitech rods it will drop straight in

Longer cut to length pushrods will be needed again unless your lucky

The barrel shims will depend on your desired compression and deck height, with a 5mm bigger stoke to retain stock compression you'd use 2.5mm spacers per side, if you wish to raise your dek and hence compression you would run smaller spacers to set your deck tighter etc

Cb have an engine calculator on their website to work it all out for you

If you get stuck drop me a pm:D

Cheers James
Click to expand...
Looks like I might be getting creative with the dremel then haha!
If I was to increase compression a little what else would this affect? I'm sure there will some kind of consequence, anything I should be wary of?

Awesome I will definitely go and have a look at that, thanks for your help, really apprecieate it!

Ryan
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Rycooled
Forsh said:
good read...


http://www.amazon.co.uk/Aircooled-Engine-Interchange-Motorbooks-Workshop/dp/0760303142

:)
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Mr.Whippy said:
Also a good read - http://johnmaherracing.com/jmr-performance-guide-cd/

Dave.
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Thanks for those too, ill definitely have a look at those!
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notenoughtime
are you stcking with standard cam or going for something a bit warmer, seeing as you are rebuilding it ;)
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Rycooled
notenoughtime said:
are you stcking with standard cam or going for something a bit warmer, seeing as you are rebuilding it ;)
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I have a scat C35 with no use as of the moment so I will probably use that! :)
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